THE DEVELOPMENT OF THE HUMAN SKULL. 



143 



The relations of the regions of the chondro-cranium thus 

 formed to the parts of the brain and to the exits of the nerves 

 are the same as those which are observed in the bones which 

 they prefigure. 



When these bones begin to be developed, some of them 

 make their appearance in the cartilage of the embryonic skull, 

 some in the perichondrium, others in the membranous roof 

 which is continuous with the perichondrium. 



A single ossification appears around the notochord in the 

 basi-occipital region, and lays the foundation for the basi- 

 occipital bone. The ex-occipitals commence as single centres of 

 ossification in the cartilage bounding the sides of the foramen 

 magnum. The supra-occipital (SO) is developed from two 

 ossifications in the cartilage above the foramen magnum, and 



:■ A 

 7Z\ 



I 



\m 



i 



c 



6 

 a-1 



■m 



'& 



N »'' 



* ' ■ 



is. ■''«! f 



■Ik- 



w ■■...■, 



0. 1 h&~~ rfJ 





■h 



E 



Fig. 58. — Successive embryonic conditions of the human head (after Ecker). — I, II, the 

 first and second cerebral vesicles. 1, 2, 3, 4, the visceral arches; a, the maxillary- 

 process ; 6, the eye ; nf, the middle nasofrontal process ; c, the lateral naso-frontal 

 process ; t, the tongue ; an, the outer part of the first visceral cleft, which eventually 

 gives rise to the external auditory meatus. 



A, at three weeks. 



B, at five weeks. 



C, at six weeks. 



D, at seven weeks. 



E, at eight weeks. 



F, outline side view of E. 



